This invention relates to methods of growing crystals with the desired crystal structure and is directed more particularly to a method of epitaxially growing SiC layers on SiC substrates.
SiC is a semiconductor material which has many advantages over other available semiconductor materials. For example, silicon carbide rectifiers which operated up to 600.degree. C have been fabricated. No known commercially available semiconductor device is capable of operating satisfactorily to such a high temperature. Additionally, SiC has been used to fabricate luminescent diodes which emit a blue light having a wavelength near to the blue end of the visible spectrum. Such diodes are not commercially available at the present time.
In addition to its excellent stability at high temperatures, SiC has a wide-energy bandgap and, consequently, has great commercial potential particularly in the electro-optical semiconductor industry. However, SiC devices are extremely difficult to fabricate because present processes require high temperatures and, due to growth of the crystal materail on a c-face, layers of different type crystal structures or polymorphs are grown. These polymorphs create hereto-junctions which seriously degrade semiconductor performance.
Semiconductor devices having at least one junction of either the p-n type or the n-p type are generally made by growing a single crystal having the desired crystal structure. The single crystal is then used as a substrate and material with the same crystal structure is grown on the substrate by various methods as described below. The material from which the crystal growth is developed may include dopant atoms of various types.
In the well-known Lely process of growing SiC crystals, SiC is first sublimed and then condensed to form single crystals which may if desired be used as substrates. This process requires temperatures in the range of from 2300.degree. to 2800.degree. C. Dopant gases may be added to the sublimed SiC to produce the desired p-n junction.
In another process for fabricating a p-n junction in SiC, dopant atoms are diffused into a SiC substrate at a temperature in the range of from 2150 to 2250.degree. C.
Still another way to grown p-n junctions in SiC is to epitaxially grow doped layers on SiC substrates. This latter process requires temperatures in excess of 1515.degree. C, the usual temperature being in the 1650.degree. to 1750.degree. C range.
All the above-described methods require temperatures in excess of 1500.degree. C. The high temperatures required by all of these processes present a number of disadvantages. First the dopant atoms tend to diffuse and smear out the boundaries between the differently doped regions. It has been foudn that there is a significant diffusion of dopant atoms above the temperature of 1500.degree. C, although below 1500.degree. C there is very little diffusion. This undesirable diffusion increases rapidly as temperature increases. Secondly, the high temperatures required make it difficult to maintain the required degree of system purity because materials used in the apparatus, such as the chamber, become involved in the process.
In growing SiC crystals, the substrate used is normally a single-crystal platelet having a hexagonal crystal structure. In the past these platelets were prepared so that the SiC to be grown on the crystal would grow on the c-face, that is along the c-axis which is perpendicular to the c-face. Because of this previous method of preparing the substrate, many problems resulted as will now be described.
SiC grows in many different crystal polymorphs, each with its own properties. Any one of these polymorphs can grow under apparently identical conditions and are formed by the stacking of silicon-carbon double layers of atoms. Each double layer may be situated in one of three positions. The sequency of stacking determines the particular polymorph structure and the stacking direction is called the crystal c-axis.
There is a cubic structure called .beta. SiC as well as many hexagonal and rhombohedral structures which are usually identified as .alpha. SiC. The structure of .beta. SiC is unstable above 1400.degree. C and the hexagonal polymorph known as 2H SiC is unstable above 400.degree. C. The Lely process usually produces .alpha. SiC polymorphs. A polymorph known as 6H SiC is the most commonly produced and is also the most stable of all the SiC polymorphs. The .alpha. SiC crystals produced by the Lely process are usually hexagonal platelets with the crystal c-axis perpendicular to the platelet. In all prior epitaxial processes, the epitaxial layer was grown on the large c-face of the hexagonal platelet; that is, in the c-axis direction. In these processes the stacking sequence frequently changes during growth thereby yielding layers of different polymorphs. This layered structure creates heterojunctions which can seriously degrade semiconductor performance, as indicated previously.